How hard it is to replace the rca input on the shindo amplifier?

Weller WES51 is a correct choice. It's a professional high quality tool sufficiently enough to work with any electronic applications.
For terminating higher gauge wires it's best not to use soldering and use crimping. The maximum power dial for this iron is enough to solder 10awg

It's done in few moves on almost all of the amps:
1. Remove chassis and locate the broken RCA 
2. Remove broken RCA input (you may note or photograph how the wires connected or simply memorize)
3. Mount replacement RCA input the way broken old one was connected
4. Put chassis back together.

No need to learn, just see how and repeat.
After 2..3 times of practicing (tops) the job will look nice, shiny and sexy.

If you use proper tools and apply proper skills to solder a wire, you won't damage plastic chassis of RCA receptacles.
To see how to solder proper you can go YouTube where you will find an arsenal of instructional videos.

I grind tip of conical shape to a sole shape and that works very very BEST.

I’m glad that actually you chose the path of learning instead of hiring someone for insubstantial problem. I know many today’s audiophiles even refuse to learn to work on elementary items, but not this time indeed. The phrase on each today’s electronic component "Only for qualified technician" appeared in 90’s operation manuals instead of detailed schematics and service manuals to keep you away from learning and keep you closer to spending indeed. WES51 is very good investment for all electronic repair needs. I use one since 2002 SAME model. That tells about the undisputed quality of a tool.
Now next step you’ll be shopping for de-soldering tool I guess.

I use the electric one with sucker bulb and found it the most useful for all applications
I would probably want to use electric one with button, but 30W is insufficient most of times.
https://www.parts-express.com/cat/desoldering-tools/520

Desoldering station is only good when you're too much of a perfectionist who want to de-solder the whole board and change all solder joints in other words waste some time.